1. Field of the Invention
The present invention relates to a dry etching method, and more particularly, to a reactive ion etching (RIE) method.
2. Description of the Related Art
The RIE method conventionally is used as a dry etching method for various materials. In manufacturing a semiconductor device, for example, the RIE method is used for etching a material of an electrode, such as a polysilicon or an SiO.sub.2 layer. The RIE method is used because it can form a very accurate pattern in materials used in manufacturing semiconductor devices.
The RIE method is performed as described below. First, a substrate having a layer to be etched thereon is placed in a vacuum chamber having a pair of parallel plate electrodes. Second, an etching gas that will etch the layer but not the substrate is introduced into the chamber. The etching gas is discharged by applying a high frequency electric power to the parallel electrodes. As a result, the layer is etched by a gas plasma generated from the discharged gas.
Besides the RIE method, a plasma etching method, an Electron Cycrotron Resonance (ECR) type dry etching, an ion beam etching method, and a photo excited etching method are known as dry etching methods. These etching methods are similar to the RIE method with regard to chemically or physically etching the etching materials. In these etching methods, a gas containing fluorine and carbon elements, such as C.sub.2 F.sub.6, CF.sub.4, or CHF.sub.3 are used for etching a silicon oxide layer.
According to the following steps, etching of an SiO.sub.2 layer is performed and a contact hole is formed for connecting electrodes or wires in a semiconductor device such as a dynamic RAM (DRAM). If CHF.sub.3 gas is used as an etching gas to etch a contact hole or a via hole in an SiO.sub.2 layer on an Si substrate, CF.sub.3 radicals (CF.sub.3) are absorbed on the surface of the SiO.sub.2 layer. The CF.sub.3 absorbed on the surface are dissociated into C and F by an ion attack. As a result, C reacts with O in the SiO.sub.2 layer to form CO gas and F reacts with Si in the SiO.sub.2 layer form SiF.sub.4 gas.
The F contained in the CF.sub.3 absorbed on the surface of the Si substrate is extracted by H to form CF.sub.x (x=0, 1, 2, 3) as a polymer. The CF.sub.x polymers function to suppress the etching of the Si substrate. Accordingly, the selective etching of SiO.sub.2 to the Si substrate can be achieved
As the integration of a semiconductor device or circuit is increased, a high selectivity of SiO.sub.2 etching to Si is required. Moreover, as the etching pattern size becomes miniaturized according to the increase of integration formation of a contact hole having a small width and a large aspect ratio, which is the ratio of hole depth to hole width, also is necessary. Unfortunately, the conventional etching methods have not been able to accurately etch high integration patterns.
One method to improve selectivity of SiO.sub.2 to Si using CF.sub.4 and H.sub.2 as an etching gas was presented in "Selective Etching of Silicon Dioxide Using Reactive Ion Etching and CF.sub.4 -H.sub.2 ", L. M. Ephrath, J. Electrochem. Soc., Vol. 124, No. 284, 1977. This RIE method was performed under conditions described below. A 150 W of a high frequency electric power was supplied to parallel plate electrodes provided in the etching chamber. A 40 m Torr of pressure and a 20 sccm (standard cubic centimeter per minute) of CF.sub.4 gas was introduced into the chamber. The H.sub.2 gas flow was changed and a gas mixture rate of CF.sub.4 and H.sub.2 was used to measure the etching rate of SiO.sub.2 and Si.
The result of the measurement is shown in a graph of FIG. 14. As is apparent from FIG. 14, with an increasing of H2 gas flow being introduced, an etching rate of SiO.sub.2 shown by curve a increased and an etching rate of Si shown by curve b decreased. Therefore, the selectivity of SiO.sub.2 to Si was improved with the increase of H.sub.2.
However, when more H.sub.2 was introduced into the chamber, the etching of SiO.sub.2 stopped. This occurred at a 18 sccm of H.sub.2 gas flow, or when a mixture ratio of H.sub.2 to the gas mixture CF.sub.4 was about 47% because a polymer was formed on the surface of SiO.sub.2 and Si. Therefore, according to this etching method, the gas mixture ratio of CF.sub.4 and H.sub.2, which can achieve enough selectivity, is very small. Moreover, the etching rate of SiO.sub.2, or the selectivity, was decreased even with a small change in the gas mixture ratio. Therefore, it is very difficult to practically use this etching method.
An etching method having a large selectivity of SiO.sub.2 to Si, by setting a pressure in a chamber less than 10.sup.-2 Torr, was disclosed in Electro Communication Society, SSP 79-69, p55, 1981. The conditions of this etching method were a 7-8 m Torr of etching gas pressure, a 20 sccm of CF.sub.4 gas flow, and a changing H.sub.2 flow so as to change the mixture ratio of CF.sub.4 and H.sub.2. The etching was made and the etching rate of SiO.sub.2 and Si was measured as shown in FIG. 15. As is apparent from FIG. 15, if the H.sub.2 gas is increased such that a mixture ratio of H.sub.2 to the gas mixture of CF.sub.4 and H.sub.2 is 70%-100%, a high selective etching of SiO.sub.2 to Si can be achieved.
However, in the conventional RIE method of etching SiO.sub.2, if the selectivity of SiO.sub.2 to Si becomes more than 30, the etching ratio in forming a small opening contact hole is rapidly decreased. This phenomenon is generally known as a micro-loading effect. The result of this phenomenon is that, if the etching is continued to form the small contact hole completely, another portion of the substrate on which a large contact hole is formed would be etched so as to also etch the underlying Si substrate.
Moreover, if a contact hole has a small opening and has a large aspect ratio, it is difficult to accurately form the small contact hole.